Cardiac rhythm management systems provide heart therapy based on sensed conditions of the heart. Irregular cardiac rhythms, known as cardiac arrhythmia, is one example of a medical condition that may be treated with a cardiac rhythm management system. Cardiac arrhythmia result in diminished blood circulation. Examples of such systems include, among other things, pacemakers or pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the treatment of congestive heart failure (also referred to as “CHF”). CHF, which can result from long-term hypertension, is a condition in which the walls of at least one side of the heart (e.g., the left side) becomes disproportionately enlarged and the heart muscle associated with the left atrium and ventricle displays less contractility. This decreases cardiac output of blood through the circulatory system which, in turn, may result in an increased heart rate and less resting time between heartbeats. The heart consumes more energy and oxygen, and its condition typically worsens over a period of time.
As one side of the heart (e.g., the left side) becomes disproportionately enlarged, the intrinsic electrical heart signals that control heart rhythm are also affected. In a normal heart, such intrinsic signals originate in the sinoatrial (SA) node in the upper right atrium, traveling through and depolarizing the atrial heart tissue such that resulting contractions of the right and left atria are triggered. The intrinsic atrial heart signals are received by the atrioventricular (AV) node which, in turn, triggers a subsequent ventricular intrinsic heart signal that travels through and depolarizes the ventricular heart tissue such that resulting contractions of the right and left ventricles are triggered substantially simultaneously.
Where one side of the heart has become disproportionately enlarged due to congestive heart failure, however, the ventricular intrinsic heart signals may travel through and depolarize the left side of the heart more slowly than in the right side of the heart. As a result, the left and right ventricles do not contract simultaneously, but rather, the left ventricle contracts after the right ventricle. This delay between right ventricular and left ventricular contractions reduces the pumping efficiency of the heart due to movement of the septal wall between right and left sides of the heart. Congestive heart failure may also result in an another symptom, that is, an overly long delay between atrial and ventricular contractions. This too-long delay between atrial and ventricular contractions also reduces the pumping efficiency of the heart.
Historically, most cardiac rhythm management systems have addressed only one or a few heart conditions, such as CHF, bradyarrhythmias or tachyarrhythmias. For example, a device tailored to treat bradyarrhythmias may be incapable of providing therapy for tachyarrhythmias.
Efforts to meet a variety of different design objectives using hardware implemented cardiac rhythm management systems have illuminated several disadvantages. In a hardware implementation, typically a state machine is designed to account for all possible pacing and sensing combinations for all known modes of operation. One disadvantage of a hardware implementation is that the design tolerates little or no changes. Such design brittleness complicates implementation of subsequent design modifications or error corrections. In addition, correct operation of a hardware implementation cannot readily be inexpensively verified.
A purely software implementation is not without problems either. In one embodiment of a software implementation of a cardiac rhythm management system, a single ripple timer is triggered and multiple compare registers are provided to trigger selected functions. The ensuing comparisons are inefficient in that processor capacity is diverted during each comparison. In addition, the processor is forced to complete various calculations to assess the status or progress of the process. This clock rollover problem further taxes processor time and draws power to execute.
What is needed is a system that is flexible to later design changes, yet draws little processing power or time. The system should also allow full testing and evaluation of a wide variety of combinations of pacing and timing events.